git/diffcore-delta.c

   1 #include "cache.h"
   2 #include "diff.h"
   3 #include "diffcore.h"
   4
   5 /*
   6  * Idea here is very simple.
   7  *
   8  * Almost all data we are interested in are text, but sometimes we have
   9  * to deal with binary data.  So we cut them into chunks delimited by
  10  * LF byte, or 64-byte sequence, whichever comes first, and hash them.
  11  *
  12  * For those chunks, if the source buffer has more instances of it
  13  * than the destination buffer, that means the difference are the
  14  * number of bytes not copied from source to destination.  If the
  15  * counts are the same, everything was copied from source to
  16  * destination.  If the destination has more, everything was copied,
  17  * and destination added more.
  18  *
  19  * We are doing an approximation so we do not really have to waste
  20  * memory by actually storing the sequence.  We just hash them into
  21  * somewhere around 2^16 hashbuckets and count the occurrences.
  22  */
  23
  24 /* Wild guess at the initial hash size */
  25 #define INITIAL_HASH_SIZE 9
  26
  27 /* We leave more room in smaller hash but do not let it
  28  * grow to have unused hole too much.
  29  */
  30 #define INITIAL_FREE(sz_log2) ((1<<(sz_log2))*(sz_log2-3)/(sz_log2))
  31
  32 /* A prime rather carefully chosen between 2^16..2^17, so that
  33  * HASHBASE < INITIAL_FREE(17).  We want to keep the maximum hashtable
  34  * size under the current 2<<17 maximum, which can hold this many
  35  * different values before overflowing to hashtable of size 2<<18.
  36  */
  37 #define HASHBASE 107927
  38
  39 struct spanhash {
  40         unsigned int hashval;
  41         unsigned int cnt;
  42 };
  43 struct spanhash_top {
  44         int alloc_log2;
  45         int free;
  46         struct spanhash data[FLEX_ARRAY];
  47 };
  48
  49 static struct spanhash *spanhash_find(struct spanhash_top *top,
  50                                       unsigned int hashval)
  51 {
  52         int sz = 1 << top->alloc_log2;
  53         int bucket = hashval & (sz - 1);
  54         while (1) {
  55                 struct spanhash *h = &(top->data[bucket++]);
  56                 if (!h->cnt)
  57                         return NULL;
  58                 if (h->hashval == hashval)
  59                         return h;
  60                 if (sz <= bucket)
  61                         bucket = 0;
  62         }
  63 }
  64
  65 static struct spanhash_top *spanhash_rehash(struct spanhash_top *orig)
  66 {
  67         struct spanhash_top *new;
  68         int i;
  69         int osz = 1 << orig->alloc_log2;
  70         int sz = osz << 1;
  71
  72         new = xmalloc(sizeof(*orig) + sizeof(struct spanhash) * sz);
  73         new->alloc_log2 = orig->alloc_log2 + 1;
  74         new->free = INITIAL_FREE(new->alloc_log2);
  75         memset(new->data, 0, sizeof(struct spanhash) * sz);
  76         for (i = 0; i < osz; i++) {
  77                 struct spanhash *o = &(orig->data[i]);
  78                 int bucket;
  79                 if (!o->cnt)
  80                         continue;
  81                 bucket = o->hashval & (sz - 1);
  82                 while (1) {
  83                         struct spanhash *h = &(new->data[bucket++]);
  84                         if (!h->cnt) {
  85                                 h->hashval = o->hashval;
  86                                 h->cnt = o->cnt;
  87                                 new->free--;
  88                                 break;
  89                         }
  90                         if (sz <= bucket)
  91                                 bucket = 0;
  92                 }
  93         }
  94         free(orig);
  95         return new;
  96 }
  97
  98 static struct spanhash_top *add_spanhash(struct spanhash_top *top,
  99                                          unsigned int hashval, int cnt)
 100 {
 101         int bucket, lim;
 102         struct spanhash *h;
 103
 104         lim = (1 << top->alloc_log2);
 105         bucket = hashval & (lim - 1);
 106         while (1) {
 107                 h = &(top->data[bucket++]);
 108                 if (!h->cnt) {
 109                         h->hashval = hashval;
 110                         h->cnt = cnt;
 111                         top->free--;
 112                         if (top->free < 0)
 113                                 return spanhash_rehash(top);
 114                         return top;
 115                 }
 116                 if (h->hashval == hashval) {
 117                         h->cnt += cnt;
 118                         return top;
 119                 }
 120                 if (lim <= bucket)
 121                         bucket = 0;
 122         }
 123 }
 124
 125 static struct spanhash_top *hash_chars(struct diff_filespec *one)
 126 {
 127         int i, n;
 128         unsigned int accum1, accum2, hashval;
 129         struct spanhash_top *hash;
 130         unsigned char *buf = one->data;
 131         unsigned int sz = one->size;
 132         int is_text = !diff_filespec_is_binary(one);
 133
 134         i = INITIAL_HASH_SIZE;
 135         hash = xmalloc(sizeof(*hash) + sizeof(struct spanhash) * (1<<i));
 136         hash->alloc_log2 = i;
 137         hash->free = INITIAL_FREE(i);
 138         memset(hash->data, 0, sizeof(struct spanhash) * (1<<i));
 139
 140         n = 0;
 141         accum1 = accum2 = 0;
 142         while (sz) {
 143                 unsigned int c = *buf++;
 144                 unsigned int old_1 = accum1;
 145                 sz--;
 146
 147                 /* Ignore CR in CRLF sequence if text */
 148                 if (is_text && c == '\r' && sz && *buf == '\n')
 149                         continue;
 150
 151                 accum1 = (accum1 << 7) ^ (accum2 >> 25);
 152                 accum2 = (accum2 << 7) ^ (old_1 >> 25);
 153                 accum1 += c;
 154                 if (++n < 64 && c != '\n')
 155                         continue;
 156                 hashval = (accum1 + accum2 * 0x61) % HASHBASE;
 157                 hash = add_spanhash(hash, hashval, n);
 158                 n = 0;
 159                 accum1 = accum2 = 0;
 160         }
 161         return hash;
 162 }
 163
 164 int diffcore_count_changes(struct diff_filespec *src,
 165                            struct diff_filespec *dst,
 166                            void **src_count_p,
 167                            void **dst_count_p,
 168                            unsigned long delta_limit,
 169                            unsigned long *src_copied,
 170                            unsigned long *literal_added)
 171 {
 172         int i, ssz;
 173         struct spanhash_top *src_count, *dst_count;
 174         unsigned long sc, la;
 175
 176         src_count = dst_count = NULL;
 177         if (src_count_p)
 178                 src_count = *src_count_p;
 179         if (!src_count) {
 180                 src_count = hash_chars(src);
 181                 if (src_count_p)
 182                         *src_count_p = src_count;
 183         }
 184         if (dst_count_p)
 185                 dst_count = *dst_count_p;
 186         if (!dst_count) {
 187                 dst_count = hash_chars(dst);
 188                 if (dst_count_p)
 189                         *dst_count_p = dst_count;
 190         }
 191         sc = la = 0;
 192
 193         ssz = 1 << src_count->alloc_log2;
 194         for (i = 0; i < ssz; i++) {
 195                 struct spanhash *s = &(src_count->data[i]);
 196                 struct spanhash *d;
 197                 unsigned dst_cnt, src_cnt;
 198                 if (!s->cnt)
 199                         continue;
 200                 src_cnt = s->cnt;
 201                 d = spanhash_find(dst_count, s->hashval);
 202                 dst_cnt = d ? d->cnt : 0;
 203                 if (src_cnt < dst_cnt) {
 204                         la += dst_cnt - src_cnt;
 205                         sc += src_cnt;
 206                 }
 207                 else
 208                         sc += dst_cnt;
 209         }
 210
 211         if (!src_count_p)
 212                 free(src_count);
 213         if (!dst_count_p)
 214                 free(dst_count);
 215         *src_copied = sc;
 216         *literal_added = la;
 217         return 0;
 218 }